A concrete mixing station may include several scraper arms which supply the same central hopper with aggregates of various kinds. The operation of the scraper arm is controlled for example by a weighing scale. When the scraper arm has emptied a predetermined and recorded weight of aggregates of a predetermined kind into the hopper, the scraper arm is moved from one pile of aggregate to another in order to deliver a different kind of aggregate to the hopper.
According to a known technique, the endless chain of the scraper arm is allowed to operate while the latter moves laterally from one pile of aggregate to another, otherwise the resistance between the inoperative chain and the aggregate when moving the scraper arm laterally would be much too great.
When the endless chain of the scraper arm is able to move only forwardly towards the hopper, which is the case, for example, when the scraper members are constituted by buckets, the scraper arm continues to deliver aggregate into the hopper when it is moving laterally. During this period, the scraper arm thus introduces into the hopper a mixture of several aggregates, which distorts the weighing. This problem is more appreciable the further apart the piles of aggregate are spaced.
An attempt has been made to obviate this drawback by constructing scraper arms provided with chains which are movable in two directions. Thus, during the periods corresponding to the lateral movements of the scraper arm, the direction of travel of the endless chain is reversed and the scraper arm members move over the piles of aggregates without loading the hopper. However, each lateral maneuver nevertheless leads to a certain degree of mixing of the piles of aggregate and the drawback continues to exist especially when the piles of aggregate are very close to each other, in view of the fact that during the short period of rearward travel, all of the scraper arm members do not have time to jettison the aggregate which they contain.